Preheating temperature during remelting

ABSTRACT

A method of welding a component is provided. The component is welded using a preheating temperature of the component below 550° C. and above 480° C. Further, a laser or a plasma is used for the welding, wherein a power of the laser or plasma is between 500 W and 900 W, and wherein a spot size of the laser beam has a diameter from 3 mm to 5 mm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2007/008706 filed Oct. 8, 2007, and claims the benefitthereof. The International Application is incorporated by referenceherein in its entirety.

FIELD OF INVENTION

The invention relates to a method of welding the surface of a Ni base,especially a single crystal (SX) superalloy substrate using a laser beamwhile preheating the substrate to an optimized temperature for thepurpose of repairing cracks.

BACKGROUND OF INVENTION

This is useful because blades are expensive. This is especially throughfor single crystalline components (SX) components.

The U.S. Pat. No. 5,374,319 teaches that the preheating temperatureduring welding lies at 760° C., preferably at a higher temperature of920° C.

After casting or after service high temperature turbine parts (e.g.turbine blades or vanes) may present surface cracks that must berepaired prior.

SUMMARY OF INVENTION

It is an object of the invention to overcome this problem.

The object is achieved by a method according to the independent claim.Further advantageous steps are listed in the dependent claims which canbe combined which each other to yield further advantages.

A laser assisted process is foreseen for the repair of cracks affectingSX turbine parts by surface local controlled laser remelting.

When SX components are laser treated, two main types of defects mightaffect the repaired zone: spurious grains and solidification cracking.

The conditions for successful SX repair on SX components requireepitaxial and columnar growth and avoiding equiaxed or misorientedcolumnar growth responsible for grain boundaries formation. To guaranteea SX structure, a precise process control that insures epitaxialcolumnar growth is essential.

Apart from the microstructure control, conditions which produce crackfree solidification constitute a prerequisite for the repair of realparts.

The rising of the temperature of the surrounding material throughpreheating constitute the most effective way to reduce the cooling rateand the cracking tendency. The preheating treatment generally used forgamma prime precipitation strengthened nickel base superalloys consistsin heating the entire weld area to a ductile temperature set above theaging temperature (˜870° C.) and below the incipient melting temperaturebut might be defined as being set in between 950° C. and 1000° C. U.S.Pat. No. 5,374,319.

Within this temperature range the thermal gradients are reduced by oneor to order of magnitude and thus increase the risk for nucleation ofspurious grains by increasing the driving force for nucleation. Theprocess window for SX solidification is thus critically reduced whichdrastically limit the use of the SX laser assisted repair.

Such high preheating temperatures also constitute a risk for the processupscale to real parts as it can trigger recrystallization of locationpresenting high dislocation density (e.g. blade roots).

The limitation inherent to the use of the preheating treatment definedin the state of the art is solved trough the definition of a preheatingtreatment balancing those two conflicting features (spurious grainnucleation and hot cracking).

The optimal preheating temperature here proposed is below 660° C. Thisparticular temperature allows reducing the yield strength of thesurrounding material and thus the associated restraint which usuallyrestrict the required shrinkage of the weld bead and lead to tensilestress build-up in the critical area while holding the driving force forspurious grain nucleation to a sufficiently low value.

The heating source employed may consist in an induction system allowinglocal heat treatment.

Taking into account the somewhat low temperature here defined the use ofinfrared lamp or defocused laser beam might be conceivable to achievethe desired preheating temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a gas turbine,

FIG. 2 shows a turbine blade,

FIG. 3 shows a combustion chamber,

FIG. 4, 5, 6 shows a component to be repaired by welding,

FIG. 7 shows a listing of superalloys and

FIG. 8, 9 experimental results.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows, by way of example, a partial longitudinal section througha gas turbine 100. In the interior, the gas turbine 100 has a rotor 103which is mounted such that it can rotate about an axis of rotation 102,has a shaft 101 and is also referred to as the turbine rotor. An intakehousing 104, a compressor 105, a, for example, toroidal combustionchamber 110, in particular an annular combustion chamber, with aplurality of coaxially arranged burners 107, a turbine 108 and theexhaust-gas housing 109 follow one another along the rotor 103. Theannular combustion chamber 110 is in communication with a, for example,annular hot-gas passage 111, where, by way of example, four successiveturbine stages 112 form the turbine 108.

Each turbine stage 112 is fowled, for example, from two blade or vanerings. As seen in the direction of flow of a working medium 113, in thehot-gas passage 111 a row of guide vanes 115 is followed by a row 125formed from rotor blades 120.

The guide vanes 130 are secured to an inner housing 138 of a stator 143,whereas the rotor blades 120 of a row 125 are fitted to the rotor 103for example by means of a turbine disk 133. A generator (not shown) iscoupled to the rotor 103.

While the gas turbine 100 is operating, the compressor 105 sucks in air135 through the intake housing 104 and compresses it. The compressed airprovided at the turbine-side end of the compressor 105 is passed to theburners 107, where it is mixed with a fuel. The mix is then burnt in thecombustion chamber 110, forming the working medium 113. From there, theworking medium 113 flows along the hot-gas passage 111 past the guidevanes 130 and the rotor blades 120. The working medium 113 is expandedat the rotor blades 120, transferring its momentum, so that the rotorblades 120 drive the rotor 103 and the latter in turn drives thegenerator coupled to it.

While the gas turbine 100 is operating, the components which are exposedto the hot working medium 113 are subject to thermal stresses. The guidevanes 130 and rotor blades 120 of the first turbine stage 112, as seenin the direction of flow of the working medium 113, together with theheat shield bricks which line the annular combustion chamber 110, aresubject to the highest thermal stresses. To be able to withstand thetemperatures which prevail there, they can be cooled by means of acoolant. Substrates of the components may likewise have a directionalstructure, i.e. they are in single-crystal four). (SX structure) or haveonly longitudinally oriented grains (DS structure). By way of example,iron-based, nickel-based or cobalt-based superalloys are used asmaterial for the components, in particular for the turbine blade or vane120, 130 and components of the combustion chamber 110. Superalloys ofthis type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP1 319 729 A1, WO 99/67435 or WO 00/44949.

The guide vane 130 has a guide vane root (not shown here) facing theinner housing 138 of the turbine 108 and a guide vane head at theopposite end from the guide vane root. The guide vane head faces therotor 103 and is fixed to a securing ring 140 of the stator 143.

FIG. 2 shows a perspective view of a rotor blade 120 or guide vane 130of a turbomachine, which extends along a longitudinal axis 121.

The turbomachine may be a gas turbine of an aircraft or of a power plantfor generating electricity, a steam turbine or a compressor.

The blade or vane 120, 130 has, in succession along the longitudinalaxis 121, a securing region 400, an adjoining blade or vane platform 403and a main blade or vane part 406 as well as a blade or vane tip 415. Asa guide vane 130, the vane 130 may have a further platform (not shown)at its vane tip 415.

A blade or vane root 183, which is used to secure the rotor blades 120,130 to a shaft or disk (not shown), is formed in the securing region400. The blade or vane root 183 is designed, for example, in hammerheadform. Other configurations, such as a fir-tree or dovetail root, arepossible. The blade or vane 120, 130 has a leading edge 409 and atrailing edge 412 for a medium which flows past the main blade or vanepart 406.

In the case of conventional blades or vanes 120, 130, by way of examplesolid metallic materials, in particular superalloys, are used in allregions 400, 403, 406 of the blade or vane 120, 130.

Superalloys of this type are known, for example, from EP 1 204 776 B1,EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949. The blade orvane 120, 130 may in this case be produced by a casting process, also bymeans of directional solidification, by a forging process, by a millingprocess or combinations thereof.

Workpieces with a single-crystal structure or structures are used ascomponents for machines which, in operation, are exposed to highmechanical, thermal and/or chemical stresses. Single-crystal workpiecesof this type are produced, for example, by directional solidificationfrom the melt. This involves casting processes in which the liquidmetallic alloy solidifies to form the single-crystal structure, i.e. thesingle-crystal workpiece, or solidifies directionally. In this case,dendritic crystals are oriented along the direction of heat flow andform either a columnar crystalline grain structure (i.e. grains whichrun over the entire length of the workpiece and are referred to here, inaccordance with the language customarily used, as directionallysolidified) or a single-crystal structure, i.e. the entire workpiececonsists of one single crystal. In these processes, a transition toglobular (polycrystalline) solidification needs to be avoided, sincenon-directional growth inevitably forms transverse and longitudinalgrain boundaries, which negate the favorable properties of thedirectionally solidified or single-crystal component.

Where the text refers in general terms to directionally solidifiedmicrostructures, this is to be understood as meaning both singlecrystals, which do not have any grain boundaries or at most havesmall-angle grain boundaries, and columnar crystal structures, which dohave grain boundaries running in the longitudinal direction but do nothave any transverse grain boundaries. This second form of crystallinestructures is also described as directionally solidified microstructures(directionally solidified structures). Processes of this type are knownfrom U.S. Pat. No. 6,024,792 and EP 0 892 090 A1.

The blades or vanes 120, 130 may likewise have coatings protectingagainst corrosion or oxidation, e.g. MCrAlX (M is at least one elementselected from the group consisting of iron (Fe), cobalt (Co), nickel(Ni), X is an active element and represents yttrium (Y) and/or siliconand/or at least one rare earth element, or hafnium (Hf)). Alloys of thistype are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 orEP 1 306 454 A1. The density is preferably 95% of the theoreticaldensity. A protective aluminum oxide layer (TGO=thermally grown oxidelayer) forms on the MCrAlX layer (as an intermediate layer or anoutermost layer).

It is also possible for a thermal barrier coating, consisting forexample of ZrO₂, Y₂O₃—ZrO₂, i.e. unstabilized, partially stabilized orfully stabilized by yttrium oxide and/or calcium oxide and/or magnesiumoxide, which is preferably the outermost layer, to be present on theMCrAlX. The thermal barrier coating covers the entire MCrAlX layer.Columnar grains are produced in the thermal barrier coating by means ofsuitable coating processes, such as for example electron beam physicalvapor deposition (EB-PVD). Other coating processes are conceivable, forexample atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermalbarrier coating may include porous grains which have microcracks ormacrocracks for improving its resistance to thermal shocks. The thermalbarrier coating is therefore preferably more porous than the MCrAlXlayer.

The blade or vane 120, 130 may be hollow or solid in form. If the bladeor vane 120, 130 is to be cooled, it is hollow and may also havefilm-cooling holes 418 (indicated by dashed lines).

FIG. 3 shows a combustion chamber 110 of the gas turbine 100. Thecombustion chamber 110 is configured, for example, as what is known asan annular combustion chamber, in which a multiplicity of burners 107arranged circumferentially around an axis of rotation 102 open out intoa common combustion chamber space 154 and generate flames 156. For thispurpose, the combustion chamber 110 overall is of annular configurationpositioned around the axis of rotation 102.

To achieve a relatively high efficiency, the combustion chamber 110 isdesigned for a relatively high temperature of the working medium M ofapproximately 1000° C. to 1600° C. To allow a relatively long servicelife even with these operating parameters, which are unfavorable for thematerials, the combustion chamber wall 153 is provided, on its sidewhich faces the working medium M, with an inner lining formed from heatshield elements 155.

A cooling system may also be provided for the heat shield elements 155and/or their holding elements, on account of the high temperatures inthe interior of the combustion chamber 110. The heat shield elements 155are then, for example, hollow and if appropriate also have cooling holes(not shown) opening out into the combustion chamber space 154.

Each heat shield element 155 made from an alloy is provided on theworking medium side with a particularly heat-resistant protective layer(MCrAlX layer and/or ceramic coating) or is made fromhigh-temperature-resistant material (solid ceramic bricks). Theseprotective layers may be similar to those used for the turbine blades orvanes, i.e. for example meaning MCrAlX: M is at least one elementselected from the group consisting of iron (Fe), cobalt (Co), nickel(Ni), X is an active element and represents yttrium (Y) and/or siliconand/or at least one rare earth element, or hafnium (Hf). Alloys of thistype are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 orEP 1 306 454 A1.

It is also possible for a, for example, ceramic thermal barrier coating,consisting for example of ZrO₂, Y₂O₃—ZrO₂, i.e. unstabilized, partiallystabilized or fully stabilized by yttrium oxide and/or calcium oxideand/or magnesium oxide, to be present on the MCrAlX. Columnar grains areproduced in the thermal barrier coating by means of suitable coatingprocesses, such as for example electron beam physical vapor deposition(EB-PVD). Other coating processes are conceivable, for exampleatmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barriercoating may have porous grains which have microcracks or macrocracks toimprove its resistance to thermal shocks.

Refurbishment means that after they have been used, protective layersmay have to be removed from turbine blades or vanes 120, 130, heatshield elements 155 (e.g. by sand-blasting). Then, the corrosion and/oroxidation layers and products are removed. If appropriate, cracks in theturbine blade or vane 120, 130 or the heat shield element 155 are alsorepaired. This is followed by recoating of the turbine blades or vanes120, 130, heat shield elements 155, after which the turbine blades orvanes 120, 130 or the heat shield elements 155 can be reused.

FIG. 4 shows a component 1, 120, 130, 155, which comprises a substrate4. This substrate 4 possesses a crack 10 or hole 10 which has to beclosed. The hole 4 or crack 10 is a blind hole. The substrate 4 ispreferably made of a superalloy, preferably listed in FIG. 7,especially: PWA1483, CMSX4.

The preheating is preferably performed only locally around the area 10to be welded, that means that the area around the crack 10 is heated andin the other regions the temperature is much lower.

Very good results have been obtained in a temperature range between 490°C. and 510° C. (FIG. 9), where good high yielding rates are reached(number of defects are low).

The preheating temperature is preferably maintained during the wholewelding process.

Especially the depth of the cracks 10 is between 0.75 mm up to 1.5 mm.The depth of the cracks 10 is up to 1 mm, very especially in the rangeof 1 mm. The width of the crack 10 at the surface 22 of the substrate 4is in the range between 10 μm to 100 μm.

Although there are several possibilities of lasers 13 as welding deviceto be used it was found that a Nd-YAG or high power diode laser type isthe best to be used. The diameter of the spot size of the laser beam isin the range of 2.5 mm to 5 mm, especially 3 mm to 5 mm and veryespecially in the range of 4 mm. At least diameters of ≧2.5 mm should beused. Surprisingly it was found that such a big diameter of the laserbeam shows good results of repairing that small cracks 10 (10 μm to 100μm), wherein “small” relates to the crack width at the surface.

The power P_(Laser) [W] of the laser 13 is between 450 Watt and 950Watt, especially 500 Watt to 900 Watt (FIG. 8), so that laserintensities of about 2.3 kW/cm² to 30 kW/cm², especially 2.5 kW/cm² to29 kW/cm² are reached. Lower laser power than 450 W leads to aninsufficient melting of the area 10 to be melted and must be avoided.The number of defects increases (FIG. 8). A higher laser power than 950W leads to a too big weld bath and even vaporization of alloyingelements, because the temperature is getting too high. Number of defectsin the weld increase (FIG. 8). The range of the laser power is very wellbalanced. Preferably the relative movement of the laser beam and thesubstrate 4 to be welded is <1 mm/s, especially ≦0.9 mm/s and especially≦0.5 mm/s and very especially 50 mm/min. Preferably the relativemovement is ≧0.4 mm/s, especially ≧0.6 mm/s.

Nevertheless, additional material 19 (FIG. 6), especially: PWA 1483SX,CMSX4 based powders can be added by a material feeder 16 (FIG. 6,especially in form of powders) whose supplied material is melted againby the welding apparatus 13.

1.-23. (canceled)
 24. A method of welding a component, comprising:welding the component; providing a preheating temperature of thecomponent below 550° C. and above 480° C.; using a laser or a plasma forthe welding, wherein a power of the laser or plasma is between 500 W and900 W, and wherein a spot size of the laser beam has a diameter from 3mm to 5 mm.
 25. The method according to claims 24, wherein the diameterof the laser beam is 4 mm.
 26. The method according to claim 24, whereinthe component comprises a nickel based superalloy, the superalloy beinga directionally solidified columnar grained alloy.
 27. The methodaccording to claim 24, wherein the component comprises a nickel basedsuperalloy, the superalloy being a single crystal superalloy.
 28. Themethod according to claim 24, wherein the component is preheated by aninduction system.
 29. The method according to claim 24, wherein thecomponent is preheated by an infrared lamp.
 30. The method according toclaim 24, wherein the component is preheated by the laser which is alsoused for welding.
 31. The method according to claim 24, wherein thecomponent is preheated only locally in an area to be welded.
 32. Themethod according to claim 24, wherein a material is added to the area tobe welded.
 33. The method according to claim 24, wherein the preheatingtemperature is about 500° C.
 34. The method according to claim 24,wherein no material is added to the area to be welded.
 35. The methodaccording to claim 24, wherein the preheating temperature is maintainedduring the whole welding process.
 36. The method according to claim 24,wherein a relative movement of the laser beam and the component is lessthan 1 mm/s.
 37. The method according to claim 24, wherein a relativemovement of the laser beam and the component is 1 mm/s.
 38. The methodaccording to claim 36, wherein the relative movement is between 0.4 mm/sand ≦0.9 mm/s.
 39. The method according to claim 36, wherein therelative movement is 50 min/min.
 40. The method according to claim 24,wherein the laser is a Nd-YAG laser.
 41. The method according to claim24, wherein the welding method is a remelting process.